1. Field of the Invention
The present invention relates to image recording apparatus, and more particularly, to recording apparatus in which images are produced on a recording medium by a scanning light beam which is modulated in accordance with information signals.
2. State of the Prior Art
In recording apparatus known as scan printers, a beam of light is modulated in accordance with image information coming from an electronic computer or the like. The modulated beam is focused on a recording medium by optical elements and is moved across the medium by a scanning device.
In certain scan printers, the beam of light is produced by a laser, and an electrophotographic process is used to record the data. In such printers, the quality of alphanumeric prints depends, among other factors, on the proper exposure of the various fonts. For example, a font formed of lines of uniform thickness, such as a Helvetica font, retains its quality through a rather wide variation in exposure. On the other hand, the quality of a font which contains lines of non-uniform thickness, e.g., a script font, is only acceptable within a much narrower range of exposures. In practice, a typical printed page contains a large variety of fonts and point sizes, and it can also include line art and half-tones. All of these types of images require a specific exposure level in order to be printed in an optimum fashion. There is a problem in known image recording apparatus in that the exposure setting must be a compromise, since the exposure setting remains the same, within the stability limits of the scanning exposure source, for an entire document.
U.S. Pat. No. 4,387,983, is directed to the problem of "thinning of the image" in the printing of certain characters on a photosensitive medium. This patent discloses apparatus in which information is recorded by scanning a photosensitive medium with a beam of light regulated by a modulation signal. When a line segment to be printed is relatively narrow, the width of the modulation signal is enlarged to avoid undesirable thinning of the recorded image. The pulse duration of the modulation signal is changed only when it is shorter than a certain predetermined pulse duration. Such a system requires complex signal processing circuits to selectively change the width of the modulation signal, and there is no disclosure in this patent of a means of insuring the proper exposure for a given font.